Table of Contents
11.2.5.2 Damage to Accessory Equipment and Peripheral Devices
Many damages and damage mechanisms concerning the accessory equipment and peripheral devices have already been discussed in Chapter 11.2.5-1 and other volumes of this work. Therefore, the following is limited to selected examples (Examples 11.2.5-1 to 11.2.5-7). The processes that are covered are those that are likely to result in special consequences for the aircraft.
Example "Defective O-ring" (Ref. 11.2.5-1):
Excerpt: “The fire warning light for the left engine illuminated while in cruise flight. The crew secured the engine and a fire bottle was discharged….Damage to the airplane was confined to fire damage to the bottom of the engine cowling. The fire was the result of a deteriorated O-Ring on the number six fuel nozzle drain collector tube.”
“… Total time on the defective O-Ring was reported as 339 hours.”
Comments: More specific information regarding the type of O-ring damage was not provided. However, this example illustrates the safety-relevant function of O-rings, and also the importance of ensuring their proper function.
Example "O-ring failure resulting in crash" (Ref. 11.2.5-2):
Excerpt: “Witnesses saw the aircraft in level flight and observed a bright orange flash near the wing root. The first flash was followed about a second later by a much larger dark orange fireball and black smoke. The right main wing then separated from the aircraft. No lightning activity was reported in the vicinity of the aircraft….Aircraft (of the same type) have a history of fuel leaks in the dry bay. The source of the leaks, flattened or pinched O-Rings, are on-condition replacement items. The aircraft was in long term storage in the desert for 2 years prior to acquisition by the operator for fire tanker duties…Emergency procedures warn of fuel leaks in this area and require inspections prior to each flight.”
The appendix states: “..According to the …(OEM of the aircraft) failures that have occurred were the result of fuel line flexing or thermal expansion. They reported that a failure is more likely to occur in a fuel line coupling as opposed to a fuel valve, although the possibility exists for both. They stated that an O-Ring failure can range from seeping or dipping occurring over time, up to a failure that results in a high pressure spray….prolonged storage…could result in the O-Rings drying out or shrinking.”
Comments: The affected aircraft was a four-engine turboprop cargo aircraft. Although the damage evidently involved a fuel line in the nacelle, the damage symptoms also apply to pipes in the engine. This is an excellent example of typical problems with O-ring seals.
Figure "Fatigue at light metal gearbox housings": Flanges used to fasten the gearbox, and the aggregates it powers (pumps, starter/generators, regulators), to the engine can be subjected to high dynamic loads (Ill. 11.2.5-6; bottom diagram). The low dynamic strength of the light-metal alloy cast housings (Al and Mg alloys) promotes dynamic fatigue. The top diagram shows fatigue cracking (HCF) in the transition from the flange neck to the gearbox cover.
Example "Pipe connections" (Ref. 11.2.5-3):
Excerpt: “Just after takeoff the aircraft experienced a fire in the number 3 engine which was extinguished by the on board fire bottles. Investigation revealed that a secondary fuel line had pulled out of a B-nut ferrule fitting at the fuel flow divider manifold which allowed the fuel to discharge into the engine cowling area. The FAA SDR data base contained a record of 5 prior occasions where either a primary or secondary fuel line had pulled out of the B-nut ferrule fitting. A factory modification to the fuel lines was in process of implementation…but had not yet been accomplished…“
Comments: Pipe connections are frequent causes of leaks. Unfortunately, the exact failure mechanism is not described here. The replacement with another modification indicates that a design-specific weakness had a considerable influence on the resulting damage.
Example "Failure mechanism of splined socket connections" (Ref. 11.2.5-4):
Excerpt: ”…During takeoff the engine lost thrust and the pilot ejected. Just after ejection the engine recovered, thrust increased and the aircraft crashed into the ground…Investigators believe that wear on an engine alternator shaft caused the alternator to slip, and the powerplant's control interpreted this as an engine overspeed. As a result, the controller kept reducing fuel to the point where thrust was severely compromised. Eventually, just after the pilot ejected, the controller determined that an overspeed was not the problem, which corrected the problem, prompting an increase in engine thrust.
`This was a known problem, and it was being addressed by an order that called for all …(fighter aircraft of the same type) with electronic controls to undergo an alternator shaft wear inspection as part of normal aircraft maintenance. We thought there was a low probability that the failure would happen during takeoff' …“
Comments: This is a typical failure mechanism of splined socket connections (see Volume 2, Chapter 6.2). This type of damage evidently occurs quite frequently in this engine model. There does not seem to be a simple remedy, for otherwise the operators would not rely on inspections to control the problem.
Figure "Weaknesses of oil tank designs": In order to realise an oil tank with the greatest possible volume while keeping the outer contour of the engine as small as possible, form-fitted flat oil tanks (schematic, depicted without connections) have come into widespread use, especially in fighter aircraft (engine in the fuselage, bottom right diagram). These oil tanks, which have flat or one-dimensionally curved walls, can become deformed by differences in internal and external pressure. Possible remedies include stiffeners in the shape of crimps or welded stiffening bars (right detail).
A further problem of flat oil tanks is their sensitivity to vibrations. It must be pointed out that stiffeners fastened to thin metal walls with roll welds or spot welds act as pronounced notches and promote dynamic fatigue cracking. This can create dynamic fatigue cracks in the outer walls of the tanks, primarily at the welds of inner metal sheets (left detail). These cracks can create oil leaks that result in oil fires. Filigreed inner metal structures such as baffle sheets and pipes are also at risk of vibrations. Vibrations must be reduced to acceptable levels through damping measures. In one case, damping mats were adhesively bonded onto large flat oil tanks (top diagram), successfully suppressing vibrations. In this configuration, the belts necessary for fastening can also be used for damping. Al alloys with the low dynamic strength typical for these materials were used in oil tanks in older engine types. These oil tanks frequently exhibited dynamic fatigue cracks. For this reason, Ti alloys and stainless steels are the preferred materials for oil tanks today.
Spherically curved oil tanks (bottom right diagram) are very resistant to vibrations. However, the drawback of this configuration is smaller capacity, since it cannot be optimally fitted to the engine contour. As a general rule: the verification of sufficient operating strength and desired operating behavior in oil tanks must be done through realistic tests, especially tests on the engine. After the tests, the inside of the oil tanks must be closely inspected for dynamic fatigue cracks.
Figure "Swelling of an oil supply hose": Accelerations (G-forces; Volume 2, Ill. 7.1.2-12) cause oil to collect in certain areas of the tank. The required intake amount must be guaranteed during all maneuvers and flight attitudes. In this case involving a very flat oil tank (middle diagram), a flexible synthetic (rubber?) corrugated tube with a heavy metallic intake head (top diagram) was used. The hose was stiffened by individual wire rings (not a continuous spiral) to prevent collapse. The insufficient strength of the hose material under the long-term influence of hot synthetic engine oil caused it to soften and swell. This , in turn, caused the inner stiffening rings to shift, lengthening the hose by several centimeters (bottom diagrams). The altered hoses were no longer able to follow the oil, and collapsed when sucking. This posed an acute threat to the engine`s oil supply system.
The bottom diagrams show a typical problem with pendular hoses. This phenomenon has been repeatedly observed in oil tanks in various engine types, and seems to be relatively common and consistent. Heavy vibrations of the oil tank, and therefore also the pendulum, caused the latter to stand up against the G-forces and rise up out of the oil. It was thus prevented from sucking up oil. Experience has shown that this effect can be reproduced successfully with an electrodynamic shaker. Proper suspension and damping of the oil tank is expected to solve the problem.
Example "Possible digital electronic engine control failure" (Ref. 11.2.5-5):
Excerpt: ”..The first …incident (on a fighter engine) occurred in December when the control logic in a digital electronic engine control failed. `This incident is strange and unique, we've never seen anything like it in 2.2 million flight hours'…A root cause analysis is still underway to determine the failure mode.”
Comments: Is this a typical failure mechanism of electronic regulators? - it is very rare, and extremely difficult to analyze and reconstruct.
Example "Secondary damage complications" (Ref. 11.2.5-6):
Excerpt: “(The aircraft manufacturer) working on an urgent redesign of the variable speed constant frequency (VSCF) generators…after a number of failures caused damage to engine mounted gearboxes…The company says:'We have found instances where the VSCF failed, resulting in damage to the gearbox. The drive shaft was designed with a shear section so if there is an internal mechanical failure to the VSCF, it will limit the damage. We are therefore redesigning the shaft to shear at a lower failure value.' The problems cause secondary damage to the gearbox and other parts on the powerplant by introducing contaminants into the engine….(the aircraft manufacturer) says `around a dozen' VSCF failures have been reported.
Comments: This example shows the danger of fouling and damage to other components in case of damage to the oil, air, or fuel systems. Unlike the fuel system, the oil system is closed, i.e. the oil circulates in a closed loop. For this reason, contaminants that are created within the system can bypass filters and damage the engine in many different ways.
Example "Failing transistor leading to large scale inspections" (Ref. 11.2.5-7):
Excerpt: “The US Federal Aviation Administration has ordered immediate inspections of …full authority digital electronic control (FADEC) systems…after the discovery of failed transistors in some units that could cause in-flight dual engine shutdowns.,,The (engine OEM) says affected production lots have been defined and identified, and the failure mode established'. It adds that most of the failures occurred in testing prior to delivery, but that some in-flight shutdowns subsequently occurred. Failures occurred because of a failed …transistor in the FADC.”
Comments: The testing of the electronics prior to delivery was evidently not suitable for safely preventing a failure during later operation. This may indicate a specific safety-relevant problem with electronic parts.